IntegerDigits

IntegerDigits[n] gives a list of the decimal digits in the integer n. 

IntegerDigits[n,b] gives a list of the base b digits in the integer n. 

IntegerDigits[n,b,len] pads the list on the left with zeros to give a list of length len. 

IntegerDigits[n,MixedRadix[blist]] uses the mixed radix with list of bases blist.

Please visit the official Wolfram Language Reference for more details and examples on core symbols.